This invention relates to a mass spectrometer and, more particularly, to a mass spectrometer capable of lowering the detection limit for those components, of the gas mixture to be examined, which are present in an only very small concentration.
From German Pat. No. 1,798,377, there is known a device comprising a cold-cathode ionization gauge, a mass spectrometer with a hot-cathode ion source, an analyzer and collector, and a vacuum vessel provided with a gas inlet. In this device, in order to purify the gas prior to entering the ion source, the ionization gauge is disposed in the gas stream before the ion source. The purpose of this known arrangement is to prevent impurities, such as grease and oil vapors or other hydrocarbons, which are frequently contained in a gas stream to be analyzed, from passing into the ion source of the mass spectrometer. Such impurities may be decomposed on the hot-cathode of the ion source and lead to carbon deposits causing disturbances in operation.
From German preliminary Pat. No. 1,698,533, there is further known a high-vacuum ion source comprising a gas inlet system, devices for producing locally concentrated ionization energy in separated ionization zones in the high-vacuum space of the arrangement, devices for producing electric field forces which are effective up to the ionization zones, and separate systems for intercepting ions which have been withdrawn from the ionization zones by the field forces. In this arrangement, the ionization devices are designed so that, in two ionization zones, the ionization energies are rated differently to an extent such that, for certain constituents of the gas mixture, in particular for the component helium and the other components of the mixture, they are associated with considerable different levels of ionization probability. This known arrangement is directed to a high-vacuum gas ion source permitting an ionization of the analysis gas by means of different ionization energies and a separate measurement of the different ion streams resulting therefrom.
In connection with a mass spectrometer, this known arrangement may be used for the analysis of a gas mixture leaving a gas-chromatographic separation column. In such a case, the stream of analysis gas is split into two partial streams by means of a branching inlet conduit whose two branches must be symmetrically arranged and formed as similarly as possible from their junction, in order to assure that the two halves of each mixture component (peaks) pass the two ionization zones approximately simultaneously. The known arrangement has the drawback, however, that it is hardly possible to make the two branches exactly equal in form to an extent such that they would influence the different components of the examined gas mixture in exactly the same manner. Already a very small difference, for example, in length and width of the two branches, or a slightly different condition of the inner wall surfaces, which may easily happen during operation, causes a stronger adsorption of one or another component in one of the conduits relative to the other one, which results in measuring errors. This is particularly troublesome if there are to be detected components which are present in the gas mixture in only a very small concentration, so that the possibility of their detection, or detection limit, is substantially reduced.